Delivering electrical pulses to a tissue such as a nerve or a muscle has been known in the art for diagnosing or treating a number of disorders such as Parkinson's disease, epilepsy, chronic pain, motor disorders, and many other applications. In its simplest form, a device for delivering such electrical pulses comprises an electrical pulse generator, stimulating electrodes and wires electrically coupling the electrodes to the electrical pulse generator. Depending on the applications, the electrodes can be applied on the skin and the electrical current transmitted transcutaneously to the tissue to be treated. For some applications, however, the electrodes must be applied directly onto the tissue to be treated, requiring the use of an implantable device. It is clear, in the latter case, that miniaturization of the implant is of paramount importance.
Depending on the tissue to be treated, the type of electrodes used, and the distance between electrodes, the voltage required between implanted electrodes is generally of the order of 15V±5V. Such voltage requires an electrical pulse generator of such dimensions that electric stimulating implants are generally formed of two separate components: on the one hand, the electrodes which are implanted directly onto the tissue to be treated and, on the other hand, the electrical pulse generator, of larger dimensions, which can be implanted at various locations in the body depending upon the application but most often in the subclavian region, the lower abdominal area or gluteal region. The wires connecting the pulses generator to the electrodes are generally coiled to provide flexibility, to permit the distance from the electrical pulse generator and the electrodes to be varied and to enhance mechanical stability with a higher compliance with respect to body movements. Because of the use of electric wires, in particular when coiled, such implants are incompatible with magnetic resonance imaging (MRI) apparatuses and also with simple metal detecting portals as used in airports, banks, and the like.
As shown in FIG. 1(b), digital-analog converters (DAC) are usually used to deliver the required current and voltage to the stimulating circuit, but few are satisfactorily accurate at voltages greater than 5V or even than 10 V, while still providing sufficient current resolution and with a power consumption compatible with an implantable device.
The Compound Action Potential (CAP) is the algebraic sum of all individual fibre action potentials of a tissue stimulated by electrodes. Recording the CAP is important because it reveals the reactivity of the tissue to the stimuli, the threshold voltage, the latency of the beginning of the CAP, distribution of fibres types making up the stimulated tissue, etc. The recording of the CAP response near the stimulation point is quite challenging because of the small amplitude of the nerve CAP (typically a few tens of μV while the stimulus is in the range of 5 to 15 volt, resulting in a large contamination of the recording input by the stimulus artefact that severely obscures the genuine CAP signal.
The electrical pulse generator in electrical stimulating implants is generally powered by a battery, either primary (non-rechargeable) or rechargeable. A rechargeable battery must be recharged at regular intervals, while the number of recharge cycles reduces the battery life and capacity, which may range from one day to several months depending on the applications and type and size of the battery. As discussed above, the battery is generally implanted remotely from the tissue to be stimulated, in places such as in the sub clavicular space, the abdominal area or the gluteal region. The battery can be recharged transcutaneously by means known in the art. For example, an implantable medical device may include a solar cell configured to provide energy to recharge a power source such as a battery. The solar cell may be implanted in the body of a host such that a surface of the solar cell is provided under a layer of translucent skin, which allows the solar cell to receive light from outside the body. Examples of such systems are disclosed in US20090326597, WO2014136022, or US20120035725.
U.S. Pat. No. 8,744,568 proposes a medical device comprising an implantable electroactive polymer which is electrically stimulated by a photovoltaic cell as a source of electrical power. This device can be used in particular for releasing a therapeutic agent in situ. This device is, however, not used for electrically stimulating a tissue.
There remains a need in the art for tissue electrical stimulating devices which are safe, reliable, have a long autonomy. It should also be compatible with MRI, allow CAP recording near the stimulated point and, for implants, being of small size. The present invention proposes such device. These and other advantages of the present invention are presented in the next sections.